Titanium metal has been essential to the aerospace industry since the early fifties because it combines a high strength to weight ratio with the ability to perform at much higher temperatures than aluminum or magnesium. It also has growing usage in the chemical processing industries because of its excellent resistance to corrosion such as, chloride corrosion.
Much of the United States primary titanium is imported from Japan and Europe. A majority of titanium is made by the "Kroll Process" which involves magnesium reduction of titanium tetrachloride, which is in turn made from rutile (TiO.sub.2). Titanium metal is also made by sodium reduction and electrowinning. The product of the "Kroll Process" is a metallic sponge which is later consolidated by a high temperature arc melting process. The most important consideration for any process making titanium is to prevent contamination with either metallic or non-metallic impurities, because even small amounts of oxygen or nitrogen can make the product brittle and unworkable, although carefully controlled amounts of oxygen, nitrogen, and carbon may be added to strengthen titanium alloys.
Titanium may be produced by reducing volatile titanium halides such as the chloride, bromide or iodide with an aluminum-zinc alloy. (See U.S. Pat. No. 2,753,256 to Olson). The Olson patent describes a vapor-liquid reaction wherein the titanium compound that is being reduced is introduced to the process as a vapor. Therefore, relatively non-volatile titanium compounds, e.g. titanium fluoride, alkali metal fluotitanates, etc. are not mentioned as suitable for reduction to titanium.
Others have reduced such relatively non-volatile titanium salts to zero valent titanium by means of molten aluminum. For example, see U.S. Pat. Nos. 2,550,447; 2,781,261; 2,931,722 and 2,837,426. Other reductants used to reduce such relatively non-volatile salts to zero valent titanium include alkali metals, (e.g. sodium), as described in U.S. Pat. Nos. 2,857,264 and 3,012,878; alkaline earth metals, as described in U.S. Pat. No. 3,825,415. None of these references teach the use of an aluminum-zinc alloy for reducing such relatively non-volatile salts, nor the multi-step reduction process described below.
Other art of interest may be found in U.S. Pat. Nos. 4,127,409 and 4,072,506 which are related to the recovery of zirconium and hafnium, respectively, by the reduction of the corresponding potassium chlorozirconates or hafniates by means of an alloy of aluminum and zinc.
In neither patent is the adiabatic reduction process of the instant invention suggested. More specifically, molten potassium chlorozirconates or chlorohafniates are mixed with the molten aluminum-zinc alloy and the resulting admixture heated to a reduction temperature of about 900.degree. C. It is obvious that the external heat input to the processes described in the aforesaid patents is necessarily great.